This invention relates to a device for cleaning substrates and for forming thin films utilizing a reactive gas.
With increasing progress of the semiconductor industry in recent years, there have been developed a variety of manufacturing methods for semiconductors. One of these methods is generally called the CVD method. The CVD method is in most cases used to form thin films on substrates by utilizing a chemical reaction at a high temperature. More specifically, polycrystalline thin films such as silicon (Si). silicon nitride (Si.sub.3 N.sub.4) or silicon dioxide (SiO.sub.2) for example, are manufactured.
FIG. 1 is a schematic view of a conventional hot-wall type thin film forming device using a depressurized CVD method which is described in Solid State Tech., Apr. 63 (1977), pp. 63-70.
In the figure, designated at reference numeral 1 is a quartz reaction tube in which are disposed a plurality of substrates 2 and a substrate fixing member 3 adapted to fix these substrates 2 in parallel in the longitudinal direction at equal intervals. Introduced to the quartz reaction tube 1 are reactive gases from gas bombs 6, 7 through gas flow rate regulation valves 8, 9, respectively. At the same time, the reacted gas within the quartz reaction tube 1 is discharged by a vacuum exhaust system 11 through a gate valve 10. A heater 5 connected to an alternating current power supply 4 is arranged around the quartz reaction tube 1 to heat the substrates 2 in the quartz reaction tube 1 up to high temperatures on the order of several hundred .degree.C. to several thousand .degree.C., so that the chemical reaction of the reactive gases is caused on the substrates 2.
In running the conventional thin film forming device thus constructed, first the vacuum exhaust system 11 is operated to discharge the gas within the quartz reaction tube 1 and the gas flow rate regulation valves 8, 9 are regulated to allow reactive gases, e.g., silane gas (SiH.sub.4) and oxygen gas (O.sub.2), as well as carrier gases. e.g., argon (Ar) and hydrogen (H.sub.2), to flow into the quartz reaction tube 1 from gas bombs 6, 7 through the gas flow rate regulation valves 8, 9, while depressurizing the interior of the quartz reaction tube 1 to the order of 0.1 mmHg to 10 mmHg. Then, the heater 5 is energized to heat the substrates 2 up to high temperatures on the order of several hundred .degree.C. to several thousand .degree.C., thereby effecting the chemical reaction (e.g.. SiH.sub.4 +20.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2 O) of reactive gases on the substrates 2. Silicon dioxide produced as a result of the above reaction is deposited on the substrates 2 to form thin films thereof.
FIG. 2 is a schematic sectional view showing a conventional device for cleaning substrates which is described in Japanese Patent Laid-Open No. 55-9947(1980). In the figure, designated at reference numeral 21 is a boat, at 22 is a filament disposed underneath the boat 21 and connected thereto, at 23, 24 are electrodes disposed above the boat 21, and at 25 is a substrate holder disposed above the electrodes 23, 24 and mounting thereon substrates (not shown). such as vapor-deposited thin films, multilayer thin films or metal substrates, to be cleaned. At 26 is a vacuum tank containing the boat 21, the filament 22, the electrodes 23, 24 and the substrate holder 25, at 27 is an exhaust system for discharging the gas within the vacuum tank 26, and at 28 is a leak valve for introducing gas to the vacuum tank 26. Incidentally, while one end of the filament 22 and the electrode 23 are connected to a power supply (not shown). the boat 21, the other end of the filament 22 and the electrode 24 are connected to the vacuum tank 26 and then grounded as shown in the Figure.
With the conventional substrate cleaning device thus constructed, first, substrates are mounted to the substrate holder 25 and the vacuum tank 26 is evacuated by the exhaust system 27 down to the range of 10.sup.-5 mmHg. Then, hydrogen gas is introduced to the vacuum tank 26 through the leak valve 28 so that the gas atmosphere is set around 10.sup.-2 mmHg. Then, when the filament 22 is energized for heating and a voltage is applied between the electrodes 23, 24, ionization discharge of the hydrogen gas occurs around the electrodes 23, 24 and the substrate holder 25 is in the area of the discharge. The substrates are bombarded by ionized hydrogen particles and the hydrogen ions reduce oxide films on the substrate surfaces for cleaning. At this time, a negative voltage may be applied to the substrate holder 25 to increase the cleaning effect.
In the thin film forming device as shown in FIG. 1, since thin films are formed on the substrates 2 through a chemical reaction under an atmosphere and at high temperatures, there arises a problem that the device cannot be applied to the formation of thin films at low temperatures where, for example, a plastic material is used as the substrate 2 and silicon dioxide (SiO.sub.2) is formed as a thin film on the surface of the plastic material. Even if such process can be performed, the rate of forming the thin films is low.
There is also a problem in that, when a carbon lubrication film is formed as the upper layer of a magnetic medium formed on a magnetic disc, for example, the previously formed magnetic medium may be destroyed because of the treatment at high temperatures.
Furthermore, in the conventional substrate cleaning device as shown in FIG. 2, if the gas pressure in the vacuum tank 26 is raised, the voltage applied between the electrodes 23, 24 is increased, or a negative voltage is applied to the substrates in order to increase the amount of ions reaching the substrates, instantaneous arc discharge is caused during the glow discharge and hence the substrates are damaged. Also, because the amount of ions reaching the substrate surface is concentrated at the center of the substrate, the device cannot be applied to uniformly cleaning a substrate of large area.